1. Field of the Invention
The invention relates to a multi-shaft screw-type extruder and in particular to a twin-shaft extruder.
2. Background Art
A screw-type extruder according to the prior art is known from DE 25 50 969 C2. It is provided with kneading disks which are non-rotatably mounted on the two screw shafts, providing for a processing of the material conveyed through the extruder, for instance homogenizing, mixing, kneading, plasticizing and the like. Depending on the number of their flights, the kneading disks may have a lenticular (two-flight kneading disk), trochoidal (three-flight kneading disk) etc. contour.
The mentioned prior art patent shows three-flight kneading disks, the crest portions of which are reduced as compared to the basic width of the kneading disk for mixing-and-scraping-studs to form on the periphery.
In addition to the mixing function as a result of the intermeshing action of the kneading disks which are disposed side by side in a joint axial position, these studs serve for scraping the material to be processed off the inside wall of the casing. To this end, the faying surfaces of the studs which extend radially outwards and are disposed by defined play towards the inside wall of the casing, slide over the inside wall of the casing, taking along the accumulated material.
A drawback of the known construction resides in that the casing wall is scraped only where the crest portion of the mixing-and-scraping-studs is fully developed, scraping not taking place in the area between these studs. This faulty scraping effect leads to considerable deterioration of the heat transfer from and to the casing wall, because the processed product is not exchanged in the areas that are not scraped. Furthermore, heat transfer by convection does not take place, owing to the bad thermal conductivity conditioned by the product. Also, these problems entail the risk that the material which has not be scraped off will scorch on the inside wall of the casing, which will result in the product being damaged by reason of the ensuing prolonged dwell times of the product in these areas. Furthermore, the technological properties of the screw-type extruder are negatively affected by the casing cross-section becoming gradually clogged.
DE 41 22 912 A1 teaches a twin-screw extruder in which the faying surfaces of the kneading disks which extend in the peripheral direction are provided with edges which are transverse to the axial direction of the screws and which at least partially slope relative to the axis of the screws. The damming effect of the kneading disks is maintained by this sloping, but their free cross-sectional passage area is enlarged to such an extent that shear peaks that might be thermally harmful to the product to be processed are clearly reduced. However, the embodiment illustrated by DE 41 22 912 A1 does not ensure either that the full surface of the inside wall of the casing is scraped.